Combine cake mix, peanut butter, eggs and water in a large bowl and mix at medium speed for 2 minutes. Fold in mini chocolate chips. Spread batter into pans and bake at 350 degrees according to pan directions (see thispostfor a chart on baking times). Cool for 10 minutes, then remove from pans and cool completely.

Cream peanut butter and butter in a large bowl. Add powdered sugar, milk and vanilla, mix at medium to low speed until creamy (you may need to add a bit more milk, depending on the consistency you need) about 1 minute.